The invention concerns intervertebral disc prostheses.
European Patent No. 0356112 discloses an intervertebral disc prosthesis comprising two plates and a cushion of compressible material interposed between and fixed to them. Such a prosthesis permits substantial mobility between the two adjacent vertebrae. It can in particular be subjected to movements of compression between the two vertebrae, and movements of torsion on three mutually perpendicular axes. Its mechanical behavior thus comes close to that of a healthy natural intervertebral disc. However, it has the particular disadvantage that the lateral stresses of each vertebra on the disc in a direction perpendicular to the longitudinal direction of the spine are transmitted in their entirety to the other vertebra, which is not the case in a real healthy disc. These shearing stresses are detrimental.
It is an object of the invention to make a disc prosthesis which more closely imitates and approximates the mechanical properties of a healthy natural intervertebral disc.
With a view to achieving this object, the invention provides an intervertebral disc prosthesis comprising two plates and a cushion interposed between the plates, the cushion comprising a compressible body which has ends in contact with the plates, where at least one of the ends is freely displaceable relative to the associated plate in a direction parallel to the plate.
By this means it is possible to avoid excessive lateral stresses between the two vertebrae in a direction perpendicular to the longitudinal direction of the spine. The behavior of a healthy natural disc is thus more faithfully reproduced.
The end or each end of the body is advantageously lodged in a recess which is provided in one of the plates and which can form a lateral abutment for this end.
It is thus possible to limit the relative lateral displacements between the body and the plate or plates.
The end or each end advantageously has a face having a zone of contact with a face of the associated plate, said faces being arranged in such a way that the zone of contact has a surface area which increases when a stressing of the plate in the direction of the body increases.
Thus, for the lowest compression values, the mechanical reaction of the prosthesis upon compression of the body varies very little as a function of the change in dimension of the body in the direction of compression. To put it another way, the curve of the mechanical reaction of the prosthesis upon compression as a function of the variation in height of the cushion is barely inclined relative to the horizontal for low values of compression, and little force is applied at the start. This property reproduces that of a healthy natural disc.
The end or each end advantageously has a face in contact with a face of the associated plate, the two faces being curved in at least one common direction and being respectively concave and convex.
Thus, after a relative lateral displacement of the body and of the plate, these two faces ensure automatic centering of these elements, repositioning them in a coaxial arrangement. When the concave face has at least one radius of curvature greater than a corresponding radius of curvature of the convex face, this additionally provides a zone of contact having a variable surface area, as mentioned above.
The body advantageously comprises a viscoelastic material, in particular silicone.
Thus, during compression, this material behaves such that the aforementioned curve has a hysteresis form, which brings the behavior of the prosthesis even closer to that of the healthy natural disc.
The prosthesis advantageously comprises a fluid interposed between the plates.
The fluid, particularly when it is compressible, further accentuates the hysteresis form.
The fluid is advantageously in contact with the plates.
The fluid advantageously extends about the periphery of the body.
The prosthesis advantageously comprises a chamber enclosing the fluid and arranged in such a way that it has a surface area of transverse section parallel to the plates which is substantially invariable when a stress moving the plates toward one another varies.
The prosthesis is advantageously intended for the lumbar region of the spine.
Other characteristics and advantages of the invention will become more apparent from the following description of the preferred embodiments given as nonlimiting examples.